Method for producing a high-pressure pipe

ABSTRACT

Method for producing a high-pressure tube with inner tube and outer tube made of metal are drawn together through a first drawing die. The outer diameter of the inner tube is smaller than the inner diameter and drawing forms a very stable frictional connection between the inner tube and the outer tube. The manufactured tube has a large wall thickness and is very robust and pressure-resistant while having a very high quality outer shell surface and in particular a very high quality inner shell surface by virtue of the cold forming process. These two properties allow a sufficiently high protection against bursting when pressures in excess of 12,000 bar are applied to the tube. The produced tube has improved dynamic pressure resistance against high pressures by combining a large wall thickness of the tube to be manufactured with a high quality inner shell surface of the tube to be manufactured.

The present invention relates to a method for manufacturing a tube.

In many technical fields, such as in injection technology for inner combustion engines and in the chemical industry, tubings are used, which are subject to high pressures and must withstand these pressures in the long term. One has to differentiate between a static, i.e. constant in time and space, and a dynamic, i.e. varying in time and/or pressure, pressure loads of the tubings. In many technical fields of application, the pressure-bearing components are exposed to a pressure load varying in time and space, partly also to a periodically pulsating pressure load, such that depending on the load level they are subject to significantly higher requirements in terms of pressure resistance when compared to components under static pressure-load. For the same design this results in the components bearing a dynamic pressure to have a faster wear. Thus, they must be replaced more often than this is the case for components under a static pressure load. The degree of wear depends on the individual stress factors such as the frequency of the pressure change and the difference between maximum pressure and minimum pressure, i.e. the pressure delta.

While the static pressure resistance of components primarily depends on the mechanical characteristics such as yield strength and tensile strength of the material used, for the dynamic pressure resistance further significant quantities have to be considered, such as the ductility (elongation) of the material, the depth of already existing cracks in the wall of the component, the degree of purity, the microstructure and the surface quality.

The failure of tubes and other components usually occurs as a result of a critical growth of a crack, for example by a crack propagating from the inner surface of the tube to the outer surface of the tube. In this process, the crack may be generated by local stress concentrations, for example at material defects in the form of lattice defects and at rough surfaces due to stress peaks, or already existing cracks continue to propagate due to the pressure load. Consequently, the wear of dynamically pressure loaded components is inter alia determined by a growth of cracks per pressure burst as well as the material characteristics, wherein a good surface quality may considerably decrease the probability of crack formation.

To achieve a high pressure resistance of tubes and other components under dynamic pressure loading, the materials of the tubes and components to be manufactured are selected to optimize the above mentioned major key factors. A subsequent improvement of the material characteristics, for example by an autofrettage also has to be considered frequently.

In order to withstand pressures beyond 15,000 bar over a longer period, the wall thickness of the tube in addition to a corresponding selection of suitable materials is a major factor for the manufacturing of the tube. The wall thickness of a tube is calculated by subtracting the inner diameter from the outer diameter of the tube. Assuming an identical material composition, the pressure resistance of the tube increases with its wall thickness, because the growth of a crack from the inner wall of the tube to the outer wall of the tube at increasing wall thickness takes a longer time. Therefore often thick-walled tubes are used in high pressure technology.

Therefore there is a need for a method for manufacturing a tube with an improved dynamic pressure resistance and an improved lifetime of the tube in the high pressure technology, respectively.

Moreover, it is an object of the present invention to provide a method for manufacturing a tube, in which an outer tube and an inner tube form a permanently stable mechanical contact with each other.

A further object of the present invention is to provide a method for manufacturing a tube with a high surface quality of the inner shell surface.

At least one of the above objects is solved by a method for manufacturing a tube comprising the steps of: providing an inner tube of metal with a first inner diameter and a first outer diameter, providing an outer tube of metal with a second inner diameter and a second outer diameter, wherein the first outer diameter is smaller than the second inner diameter, inserting the inner tube into the outer tube such that the inner tube extends within the outer tube, drawing the inner tube and the outer tube together through a first drawing die, wherein the second inner diameter is reduced such that a frictional connection is established between the outer tube and the inner tube.

A typical method for forming of individual metal tubes is drawing of these profiles through a drawing die, which usually is carried out on a drawing bench. A workpiece is clamped at a beginning of the workpiece by a clamping device and drawn through the drawing die. While the drawing die defines the outer diameter of the workpiece after forming, for the forming of hollow, longitudinally elongated workpieces, i.e. in particular tubes and hollow profiles, an additional second tool may be provided adjacent to the drawing die, which is disposed within the workpiece to be formed. Such a second tool for cold forming by drawing is either a flying plug or a mandrel or the plug or mandrel is supported on a bar inside the workpiece as an inner drawing tool.

The principle of a drawing machine is based on the fact that a workpiece to be formed is drawn through the drawing die from the discharge side, wherein the workpiece prior to forming has a larger diameter than the drawing die. Consequently, it is necessary that on the discharge side, which is behind the drawing die when considered in the drawing or motional direction, a tractive force can be applied to the workpiece. For this purpose, the drawing machine has a drawing device with a clamping device being mechanically powered relatively to the drawing die. During the drawing the clamping device clamps the work piece and initiates a tractive force.

According to the present invention, an inner tube is inserted into an outer tube such that the inner tube extends within the outer tube, and the inner tube and the outer tube are drawn together through a drawing die of a drawing bench. This drawing die in the context of the present invention is referred to as the first drawing die. In an embodiment, no inner tool, such as a plug or a mandrel, is provided, such that in this so-called hollow drawing only the outer diameter of the tube to be manufactured is reduced and smoothed. Thus, the wall thickness of the tube experiences a reduction in absolute terms, but without causing any influence on the inner surface of the tube to be manufactured.

However, the drawing process through the drawing die has an effect not only on the second outer diameter, i.e. the outer diameter of the outer tube, but also on the second inner diameter, i.e. the inner diameter of the outer tube, in the form of a reduction. In addition, a frictional connection of the outer tube to the inner tube is caused by the forces occurring during drawing through the drawing die.

Frictional connections are formed basically by the application of forces, for example in the form of compressive forces or frictional forces. In this case, the cohesion of the frictional connection is purely ensured by the acting force.

The method according to the present invention has several advantages over the methods known from the prior art:

-   (1) During drawing of the inner tube and the outer tube together     through the drawing die a frictional connection between these two     tubes is caused, which prevents a displacement of the inner and     outer tube in an axial direction, while a from fit is provided in     the radial direction. A permanently stable and intact connection     between the inner tube and outer tube is of major importance in     particular during application of such a so-called double-walled tube     in the high-pressure technique. Once this permanently stable and     intact connection fails, for example, due to an existing air gap or     clearance between the inner tube and outer tube or due to a lack of     frictional connection between the two tubes, this may result in the     double-walled tube bursting more easily at a high pressure load due     to a propagation of cracks. -   (2) The manufacturing of a double walled tube, i.e. a tube     consisting of an outer tube and an inner tube, according to the     present invention results in that the manufactured tube on the one     hand has a sufficiently large wall thickness and on the other hand     at the same time has a very high surface quality of the outer shell     surface and in particular of the inner shell surface. Only     fulfillment of these two requirements allows a sufficient level of     protection of the tube to be manufactured against a burst when the     tube is subject to pressures beyond 12,000 bar. The large wall     thickness which results from the sum of the wall thicknesses of the     outer tube and the inner tube after drawing through the first     drawing die, increases the pressure resistance of the tube, since     cracks on the inner shell surface must propagate over a longer     distance until they have reached the outer shell surface. In     addition, the frictional connection between the inner and outer     tubes ensures that these cracks do not extend along the contact     surface between the inner and outer tubes, which in the worst case     bring the inner tube to rupture. In addition, the drawing of the     inner tube and the outer tube together in an embodiment is carried     out in the cold state of the raw material, i.e. in the cold state of     the tube fed into the process. Thereby, the strength as well as the     dimensional quality and surface quality of the tubes to be     manufacture is increased compared with methods in which the tubes to     be processed are heated. Temperature gradients due to an     inhomogeneous heating of the tube which lead to a distortion of the     metal microstructure of the tube, and material defects or lattice     defects, which expand due to the temperature increase, do not occur     during a cold forming of tubes. Thus, by cold forming in a drawing     bench highly precise tubes of high surface quality can be     manufactured. However, the latter only up to a predetermined maximum     wall thickness. Once the wall thickness of the tube to be drawn     exceeds this maximum wall thickness, the surface quality decreases     drastically. Because according to the invention the inner tube and     the outer are manufactured separately from each other before they     are drawn together through the drawing die, it is possible to     manufacture the inner shell surface of the inner tube with a high     surface quality and still to provide the wall thickness of the tube     required for a high pressure resistance. Accordingly, the method     according to the present invention is particularly suitable for     manufacturing a tube with a significantly improved dynamic pressure     resistance under high pressures by the combination of the     requirements of a large wall thickness of the tube to be     manufactured on the one hand and a high surface quality of the inner     surface of the tube on the other hand.

In an embodiment of the present invention, the tool diameter of the forming inner surface of the first drawing die, the second outer diameter of the outer tube, the second inner diameter of the outer tube, the first outer diameter of the inner tube and the first inner diameter of the inner tube are selected such that the first inner diameter of the inner tube during drawing of the outer tube and the inner tube extending in the inner tube together through the first drawing die is reduced by at most 5%.

This upper limit of 5% results from the fact that in case this value is exceeded an unwanted change in the sense of a deformation of the inner shell surface of the inner tube occurs.

The contour of the change of the inner tube when considered in cross-section in at least one position in the longitudinal direction of the inner tube deviates by at most 5% from the average value of the first inner diameter. In an embodiment, the contour of the change of the inner tube when considered in cross-section in at least one position in the longitudinal direction of the inner tube deviates by at most 3% from the average value of the first inner diameter. In another embodiment, this deviation is at most 1%.

Such a requirement regarding the tool diameter, as well as regarding the first and second outer diameter and inner diameter of the outer tube and the inner tube ensures that the frictional connection caused by the drawing of the outer tube and inner tube extending within the outer tube has almost no, or at best no influence at all on the surface quality of the inner shell surface of the inner tube. This is important in so far as that the high surface quality of the inner shell surface of the inner tube as a result of drawing through the first drawing die is not to be adversely affected. In this case, the free choice of the individual above mentioned diameters is advantageous as the frictional connection caused by the drawing through the first drawing die is selectably adjustable. Thus, the frictional connection can be optimally adapted to the respective material characteristics of the inner tube and the outer tube. This in turn makes it possible to achieve the desired characteristics of the double-walled tube, like a permanently stable and intact connection between the inner tube and the outer tube as a result of the frictional connection and a high surface quality of the inner shell surface of the inner tube, and thus an enhanced dynamical pressure resistance, depending on the respective starting conditions of the inner tube and outer tube used.

In an embodiment of the present invention, the drawing of the outer tube and inner tube extending therein together through the first drawing die reduces the first outer diameter of the inner tube by at least 0.01 mm and at most 0.3 mm. The value of the reduction of the first outer diameter of the inner tube is a measure of the resultant frictional connection. The larger the reduction of the first outer diameter of the inner tube is, the stronger is the obtained frictional connection between the inner tube and the outer tube.

In a further embodiment of the method according to the invention the inner tube after drawing of outer tube and inner tube extending therein together through the first drawing die is strain-hardened and has a tensile strength of at least 900 N. In an embodiment, the inner tube after the common drawing has a tensile strength of at least 1050 N. The tensile strength is understood as the maximum mechanical tension which the material of the inner tube can withstand before it breaks or ruptures.

The strain hardening of the inner tube according to the method of the present invention can already be carried out during the manufacturing of the inner tube, i.e. before the common drawing through the first drawing die. However, the strain hardening of the inner tube can also be carried out as a result of the common drawing of the outer tube and the inner tube together through the first drawing die or occur as a result of a combination of the manufacturing of the inner tube and the common drawing. By strain hardening generally very tight dimensional tolerances and good surface qualities are achieved compared to hot forming processes. The existing strain hardening of the inner tube after drawing of the outer tube and the inner tube together through the first drawing die has the advantage that the strain hardening results in an increase of the material strength. This can be explained by the dislocation density increasing by the plastic deformation and consequently, by the dislocations interfering with each other during their motion. This increases the yield strength and the strength. In order to deform the inner tube further after strain hardening a significantly larger tension is required. This is particularly advantageous if the tube manufactured according to the method according to the present invention is exposed to high pressures, since the strain hardening of the inner tube leads to a significant increase of the pressure resistance. Thus, the tube to be manufactured according to the present invention is in particular suitable for application under high pressures.

In an embodiment of the present invention, the first outer diameter of the inner tube prior to the drawing of the outer tube and the inner tube extending therein together through the first drawing die is in a range of 6.25 mm to 6.45 mm and, after the common drawing is in a range of 6.08 mm to 6.28 mm, wherein the first outer diameter (D2) of the inner tube is reduced by the common drawing.

In a further embodiment of the present invention the tube to be manufactured after drawing of the inner tube and the outer tube together through the first drawing die has a wall thickness which is defined as half the difference between the second outer diameter of the outer tube and the first inner diameter of the inner tube, of at least one third of the outer diameter of the outer tube. Such a wall thickness provides a large stability of the tube to be manufactured and thus makes it possible to use the tube to be manufactured as a high-pressure tube, such that it can withstand pressures beyond the 10,000 bar without bursting.

In an embodiment of the present invention, the wall thickness of the tube to be manufactured is at least as large as the inner diameter of the inner tube.

In an embodiment of the method according to the present invention the tool diameter of the shaping inner surface of the first drawing die is by at least 5% smaller than the second outer diameter of the outer tube prior to the drawing of the inner tube and the outer tube together through the first drawing die. In an embodiment, the tool diameter of the forming inner surface of the first drawing die is at least 7% smaller than the second outer diameter of the outer tube prior to the drawing of the inner tube and the outer tube together through the first drawing die. In another embodiment this tool diameter is at least 10% smaller than the second outer diameter of the outer tube prior to the drawing of the inner tube and the outer tube together through the first drawing die.

In order for a frictional connection between the inner tube and the outer tube to be formed at all, it is a necessary prerequisite that the tool diameter of the forming inner surface of the first drawing is smaller than the second outer diameter of the outer tube prior to the common drawing. In this case, the percentage of deviation of the tool diameter of the forming inner surface of the first drawing die from the second outer diameter of the outer tube provides a measure of the generated frictional connection. Assuming a constant drawing speed, a stronger frictional connection is achieved the larger this percentage of deviation is. If the percentage of deviation of the tool diameter of the forming inner surface of the first drawing die is too small, there is no frictional connection and also the reduction of the second outer diameter of the outer tube is small. However, in an embodiment, there is also an upper limit for the percentage of deviation, which is reached once the first inner diameter of the inner tube changes by more than 5% both in the longitudinal direction as well as in the circumferential direction of the inner tube. In another embodiment, this upper limit is reached at a corresponding change by more than 3%, while in another embodiment, the upper limit is even reached at a corresponding change by more than 1%. By this means a negative impact of frictional connection to the high surface quality of the first inner diameter shall be excluded.

In an embodiment of the present invention, the inner shell surface of the inner tube has a surface quality such that cracks already existing on the surface do not exceed a depth of 50 μm. In an embodiment of the method according to the present invention, the cracks already existing on the inner shell surface of the inner tube do not exceed a depth of 20 μm, and in another embodiment, the cracks do not exceed a depth of 10 μm. Typical values achievable are even at a maximum depth of existing cracks of 7 μm. Such a high surface quality ensures that existing cracks hardly propagate from the inner shell surface of the inner tube towards the first outer diameter of the inner tube, such that the inner tube has a high pressure resistance.

In an embodiment of the present invention the inner tube is manufactured by drawing of a hollow of metal through a second drawing die and over a second inner drawing tool. The inner drawing tool can either be a fixed mandrel or mandrel bar. In an embodiment, the hollow undergoes some rounds of drawing over a mandrel or over a flying plug, wherein the blank of the tube to be manufactured is denoted a hollow. By multiple drawings the accuracy of the dimensions of the inner diameter and the outer diameter of the hollow, and thus of the wall thickness defined by the difference between the outer diameter and the inner diameter, as well as the surface quality of the inner and outer shell surface is improved. Using lubricants and drawing oils between the hollow and the inner drawing tool can reduce the dynamic friction occurring between the hollow to be drawn and the inner drawing tool, which contributes to a more homogeneous drawing speed. As a result, the use of drawing oils in addition leads to a low surface roughness of the tube to be manufactured.

In an alternative embodiment of the present invention, the inner tube is formed in a pilger mill by rolling a hollow of metal over a mandrel. Preferably, in the pilger mill is a cold pilger mill.

The most common method for reduction of tubes is known as cold pilger milling. The hollow during rolling is pushed over a calibrated, i.e. the mandrel has an inner diameter of the finished tube, tapered mandrel and thereby is spanned from its outside by two calibrated, i.e. defining the outer diameter of the finished tube, and rolled in the longitudinal direction over the mandrel. The hollow during cold pilger milling at a small rotation experiences a stepwise feed towards and over the mandrel, while the rollers are rotationally reciprocating horizontally over the mandrel and thus over the hollow. The horizontal motion of the rollers is thereby determined by a roll stand, on which the rollers are rotatably mounted. The roll stand at the known cold pilger rolling mills is moved back and forth by a crank shaft, while the rollers obtain their rotational motion from a gear rack being fixed relatively to the roll stand, wherein gear wheels fixedly mounted on the roller axis comb with the gear rack.

The cold pilger milling method is more complex than the drawing of a hollow through a drawing die, but by cold pilger milling tubes with extraordinarily precise dimensions of the outer and inner diameter of the tube to be manufactured can be obtained. Also in cold pilger milling lubricants, so-called mandrel lubricants, can be used between the hollow and the mandrel to reduce the sliding friction occurring and to obtain a smoother surface of the inner shell surface of the inner tube to be manufactured.

In an embodiment of the present invention, the inner drawing tool or the mandrel are made of steel with a polished surface, such that the inner shell surface which is spanned by the first inner diameter and by the length of the inner tube, during drawing of the inner tube over the inner drawing tool or during rolling over the mandrel is burnished. During the burnishing, also called smooth rolling, a highly hardened, polished inner drawing tool or a highly hardened, polished mandrel is pressed by a great force onto the surface of the inner tube to be manufactured and rolls off there. By this process a high compressive stress is generated in the roughness peaks, i.e. in the maxima of surface roughness, which causes a plastic deformation of the roughness peaks. The burnished inner shell surface of the inner tube is characterized by a low surface roughness, an enhanced dimensional accuracy and an increase in hardness, i.e. a solidified surface. These characteristics are of major importance for the resistance against high pressures beyond 15,000 bar, since these characteristics reduce the formation and growth of cracks significantly.

In an embodiment of the method according to the invention, a material of at least the inner tube or the outer tube is chosen from a group consisting of a carbon steel, a low-alloy steel and a high-alloy steel or a combination thereof. In an embodiment, the material is a high-alloy steel. In another embodiment, at least the inner tube or the outer tube is made of HP 160.

In the high-pressure technology different metal materials are used for the manufacturing of tubes and other components. For this purpose mainly unalloyed steels, low-alloy steels and high-alloy steels are used. A particularly high dynamic pressure resistance is achieved by tubes or other components of high-alloy steel, which have been strain hardened or heat treated and then burnished. θ is nitrogen alloyed austenitic stainless steel with a high strength which when compared to the standard materials has an improved corrosion resistance, a high degree of purity, a good formability and an ability to treat this material by autofrettage up to 12,000 bar. Due to its chemical composition and its high purity HP 160 has a very good resistance to intergranular corrosion and hydrogen embrittlement. The high molybdenum content provides good resistance to pitting and contact corrosion and stress corrosion cracking. Accordingly, HP 160 is a preferred material for the production of tubes with high dynamic pressure resistance.

In an embodiment of the present invention, the inner tube is corrosion-resistant. This feature is advantageous for use in high-pressure technology, as a beginning of corrosion, i.e. a progressive degradation, of the inner tube would negatively affect the pressure resistance.

In an embodiment of the present invention, the inner tube and the outer tube consist of the same material. Due to the frictional connection of the inner tube to the outer tube caused by drawing together through the first drawing die this causes the inner tube and outer tube to form a very stable connection with each other. Under microscopic consideration and in case of identical materials there is the same lattice structure of the metal microstructure in the inner tube as well as in the outer tube, such that the lattice structures can well be combined with each other on the outer shell surface of the inner tube and on the inner shell surface of the outer tube.

In an alternative embodiment of the present invention, the inner tube and the outer tube are made of different materials. This provides the advantage that there may be different material characteristics on the inner surface and on the outer surface of the tube to be manufactured as required by the technical application. In order to withstand high pressures under dynamic pressure load, the inner tube in an embodiment should be made of highly pressure-resistant and corrosion-resistant materials such as HP 160.

If the inner tube is made of a highly pressure-resistant and corrosion-resistant material, the outer tube when compared to the inner tube can be made of a less pressure-resistant and corrosion-resistant material. This allows a saving of manufacturing costs at a still very high quality of the tube to be manufactured.

Further advantages, features and possible applications of the present invention will become apparent from the following description of an embodiment and the accompanying figures.

FIG. 1a shows a schematic cross-sectional view of an outer tube and an inner tube extending in the outer tube prior the common drawing through a first drawing die according to an embodiment of the method according to the present invention.

FIG. 1b shows a schematic cross-sectional view of a tube manufactured after common drawing through a first drawing die according to an embodiment of the method according to the present invention.

FIG. 2 illustrates a schematic sectional view in the longitudinal direction of a drawing bench for manufacturing a tube according to an embodiment of the method according to the present invention.

FIG. 3 shows a schematic sectional view in the longitudinal direction of a drawing bench for manufacturing an inner tube according to an embodiment of the method according to the present invention.

In FIG. 1a schematic cross-sectional view of an outer tube 3 and an inner tube 2 extending in the outer tube 3. The inner tube 2 has a first inner diameter D1 and a first outer diameter D2, while the outer tube 3 has a second inner diameter D3 and a second outer diameter D4, wherein the first outer diameter D2 is smaller than the second inner diameter D3.

The inner tube 2 consists of the material HP 160, a high strength nitrogen-alloyed austenitic stainless steel of high corrosion resistance. In this case, the inner tube 2 has been manufactured in a pilger rolling mill by rolling a hollow over a mandrel of metal. To perform the rolling motion, there are two rolls rotatably mounted on shafts in a roll stand, which carries out a reciprocating motion. The rolls are driven by the reciprocating motion of the roll stand. The hollow arranged between the rotating rolls is rolled over a tapered mandrel and experiences a stepwise feeding after each rolling process.

The inner tube 2 prepared in this way is characterized by very precisely determinable dimensions of the first inner (D1) and outer diameter (D2) and in particular by a high surface quality on its inner shell surface 8. The surface quality of the inner shell surface 8 of the inner tube 2 has also been improved my the mandrel being made of high quality steel with a polished surface, and thus the inner shell surface 8 of the inner tube 2 is burnished during rolling. This has the consequence that cracks existing on the inner shell surface 8 have a maximum depth of only 7 μm.

The inner tube 2 shown in FIG. 1a comprises a first inner diameter D1 of 1.6 mm and a first outer diameter D2 of 6.3 mm.

The outer tube 3 in FIG. 1a consists, like the inner tube 2 of the material HP 160 and thus is also characterized by high corrosion resistance and pressure resistance. The manufacturing of the outer tube 3 was also carried out in a cold pilger mill with a burnishing mandrel, such that also the outer tube 3 has a high surface quality and very precise dimensions with respect to the second inner (D3) and outer diameter (D4).

The second inner diameter D3 of the outer tube 3 shown in FIG. 1a is 6.6 mm and the second outer diameter D4 of the outer tube is 15.9 mm. Here, the second inner diameter D3 of the outer tube 3 is configured such that the inner tube 2 may extend longitudinally in the outer tube 3, as is apparent from the cross-sectional drawing of FIG. 1 a.

FIG. 1b shows the same schematic cross-sectional view of an outer tube 3′ and of an inner tube 2′ extending in the outer tube 3′ as FIG. 1a , but in contrast to FIG. 1a after the outer tube 3 has already been drawn together with the inner tube 2 through a first drawing die 4 a of FIG. 2. By the common drawing through the first drawing die 4 a, the inner tube 2′ and the outer tube 3′ have formed a very stable frictional connection 6 with each other.

This frictional connection occurs in particular due to a suitable choice of the dimensions of the first drawing die 4 a as well as the outer tube 3 and inner tube 2. The tool diameter D5 of the forming inner surface of the first drawing die 4 a for the drawing die 4 a shown in FIG. 2 is 14.5 mm and has been selected such that by the drawing of the outer tube 3 and inner tube 2 together through the first drawing die 4 a a frictional connection 6 of the outer tube 3 to the inner tube 2 is sufficiently large to provide a permanently stable connection between the inner tube 2′ and outer tube 3′. However, the frictional connection 6 formed should not exceed a predetermined upper value, in order for the first inner diameter D1 of the inner tube 2′ not to be negatively affected in form of a change of the inner shell surface of the inner tube 2′.

In the cross-sectional view illustrated in FIG. 1b the frictional connection between the inner tube 2′ and the outer tube 3′ established by drawing did not negatively affect the first inner diameter D1 of the inner tube 2 and thus the surface quality of the inner surface 8′ of the inner tube 2′, such that the first inner diameter D1 of the inner tube 2′ after of cold forming in the drawing bench with the first drawing die has a value of still 1.6 mm. In contrast, the first outer diameter D2′ was reduced from an initial value of 6.3 mm to a value of 6.15 mm by drawing of the outer tube 3 and inner tube 2 together. Due to the frictional connection 6 between the outer shell surface of the inner tube 2′ and the inner shell surface of the outer tube 3′ the second inner diameter D3′ of the outer tube 3′ also after the drawing of the outer tube 3 and inner tube 2 together is at least substantially 6.15 mm. The second outer diameter D4′ of the outer tube 3′ after the drawing of the outer tube 3 and inner tube 2 together has the same value as the tool diameter D5 of the first drawing die 4 a, which in the drawing of FIG. 1b corresponds to a value of 14.5 mm. The wall thickness of the tube 1 manufactured, whose cross section is depicted in FIG. 1b , hence corresponds to 12.9 mm.

FIG. 2 shows a schematic cross-sectional view in the longitudinal direction of a drawing bench for manufacturing a tube 1 according to an embodiment of the method according to the present invention. During the drawing through the drawing bench shown in FIG. 2 an outer tube 3 and an inner tube 2 extending therein according to the cross-sectional drawing of FIG. 1a are drawn together through a first drawing die 4 a, wherein the drawing direction 7 in FIG. 2 is denoted by the arrow. Due to the absence of an inner tool, in this so-called hollow drawing merely the outer diameter of the tube 1 is reduced and smoothed. The wall thickness of the manufactured tube 1 thus experiences a reduction in absolute terms, but without causing an influence on the inner shell surface of the tube to be manufactured.

During the drawing of the outer tube 3 and inner tube 2 together through the first drawing die 4 a a combined force is applied consisting of a linearly acting tensile force and a radially acting compressive force on the outer tube 3 with the inner tube 2 extending therein. In this case, the tool diameter D5 of the first drawing die 4 a is selected such that the frictional connection between the outer shell surface of the inner tube 2 and the inner shell surface of the outer tube 3 resulting from the applied combined force is sufficiently large, such that a lasting and stable connection between these two surfaces is provided. Therefore, after the common drawing the inner tube 2 and outer tube 3 form such a rigid connection, that a so-called double-walled tube is formed, as shown on the left side of FIG. 2 and denoted by reference number 1. In addition, the inner tube 2′ is strain-hardened by the common drawing and has a tensile strength of 1100 N.

However, an upper limit of the frictional connection is not exceeded in order to avoid negative influences of the frictional connection to the surface texture of the inner surface 8′ of the inner tube 2′ so that the latter remains virtually unchanged or unaltered. Thus, the inner surface and the inner diameter of the tube 1 experience almost no or no change at all by the drawing process according to the present invention.

FIG. 2 also makes it clearly evident that the first inner diameter D1 of the inner tube 2 as a result of the drawing of the outer tube and the inner tube together through the first drawing die 4 a does not change while the first outer diameter D2 of the inner tube 2, the second inner diameter D3 of the outer tube 3 and the second outer diameter D4 each have undergone varying degrees of reduction by the common drawing. Due to the effected frictional connection 6 between the outer shell surface of the inner tube 2 and the inner shell surface of the outer tube 3, the first outer diameter D2 of the inner tube and the second inner diameter D3 of the outer tube are approximately equal.

FIG. 3 shows a schematic cross-sectional view in a longitudinal direction through one embodiment of a drawing bench for manufacturing an inner tube 2 according to the method of the present invention. The drawing bench in FIG. 3 in addition to a second drawing die 4 b has an inner drawing tool 5 fixed by means of a core rod not shown in FIG. 3, over which the inner tube 2 to be formed is drawn in the drawing direction 7 along the illustrated arrow. In this way, both the outer diameter D2 of the inner tube 2 and the inner diameter D1 of the inner tube 2 and the wall thickness of the inner tube 2 are reduced and brought into a narrow tolerance range.

In contrast to the drawing bench shown in FIG. 2 during manufacturing of the inner tube 2 during presence of an inner drawing tool 5, namely a drawing core, a more precise fitting of the inner diameter D1 of the inner tube 2 to the desired value occurs. Here, the value of the inner diameter D1 of the inner tube mainly results from the diameter of the inner drawing tool 5. Furthermore, the sliding of the blank over the inner drawing tool 5 results in a smoothing of the inner shell surface of the inner tube 2. The smoothing is further enhanced by use of a lubricant or drawing oil, which reduce the sliding friction between the tube to be drawn and the respective drawing tools and thus lead to a more homogeneous drawing speed.

The drawing core 5 shown in FIG. 3 has a maximum diameter of 1.6 mm, while the diameter of the second drawing die 4 b is 6.3 mm. Accordingly, the inner tube 2 shown on the left hand side of FIG. 3 has a first inner diameter D1 of approximately 1.6 mm and a first outer diameter D2 of approximately 6.3 mm.

For the purpose of original disclosure it is noted that all features as they will become apparent to a person skilled in the art from the present description, the drawings and the claims, even if they have been specifically described only in combination with certain other features, both individually and in any combinations can be combined with other features or groups of features disclosed herein, as far as the combination had not expressly been excluded or technical conditions make such combinations impossible or meaningless. A comprehensive explicit description of all conceivable combinations of features is only omitted for brevity and readability.

While the invention has been shown and described in detail in the drawings and the foregoing description, this illustration and description is merely exemplary and is not intended as a limitation of the scope of protection as it is defined by the claims. The invention is not limited to the disclosed embodiments.

Modifications of the disclosed embodiments will be apparent to the person skilled in the art from the drawings, the specification and the appended claims. In the claims the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude their combination. Reference numbers in the claims are not intended to limit the scope of protection.

LIST OF REFERENCE NUMBERS

-   D1 first inner diameter -   D2 first outer diameter -   D3 second inner diameter -   D4 second outer diameter -   D5 diameter of the first drawing die -   D5′ diameter of the second drawing die -   1 tube -   2,2′ inner tube -   3,3′ outer tube -   4 a first drawing die -   4 b second drawing die -   5 inner drawing tool -   6 frictional connection -   7 drawing direction -   8, 8′ inner shell surface of the inner tube 

1. A method for manufacturing a tube comprising the steps of: providing an inner tube made of metal having a first inner diameter and a first outer diameter; providing an outer tube made of metal with a second inner diameter and a second outer diameter, wherein the first outer diameter is smaller than the second inner diameter, and wherein inserting the inner tube into the outer tube such that the inner tube extends within the outer tube; and drawing of the inner tube and the outer tube together through a first drawing die, wherein the second inner diameter is reduced such that a frictional connection of the outer tube and the inner tube is established.
 2. The method according to claim 1, wherein the tool diameter of the forming inner surface of the first drawing die, the second outer diameter of the outer tube, the second inner diameter of the outer tube, the first outer diameter of the inner tube and the first inner diameter of the inner tube are selected such that the first inner diameter of the inner tube during the drawing of the outer tube and inner tube extending therein together through the first drawing die is reduced by at most 5%.
 3. The method according to claim 1 wherein the first outer diameter of the inner tube by drawing of the outer tube and the inner tube extending therein together through the first drawing die is reduced by at least 0.01 mm and by at most 0.3 mm.
 4. Method according to claim 1, wherein the inner tube after drawing of the outer tube and the inner tube extending therein together through the first drawing die is strain-hardened and has a tensile strength of at least 900 N.
 5. The method according to claim 1, wherein the tube to be manufactured after drawing of the inner tube and the outer tube through the first drawing die has a wall thickness, which is defined by half the difference between the second outer diameter of the outer tube and the first inner diameter of the inner tube, at least one third of the outer diameter of the outer tube, wherein the wall thickness is at least one third of the outer diameter of the outer tube.
 6. The method according to claim 1, wherein the tool diameter of the forming inner surface of the first drawing die prior to drawing of the inner tube and the outer tube together through the first drawing die is by at least is 5% smaller than the second outer diameter of the outer tube prior to drawing.
 7. The method according to claim 1, wherein the inner shell surface of the inner tube has a surface quality such that existing cracks on the surface do not exceed a depth of 50 μm.
 8. The method according to claim 1, wherein the inner tube is manufactured by drawing a hollow of metal through a second drawing die and over an inner die tool.
 9. The method according to claim 1, wherein the inner tube is manufactured by in a pilger mill by rolling a hollow of metal over a mandrel.
 10. The method according to claim 2, wherein the inner die tool or the mandrel is made of steel with a polished surface, such that an inner shell surface of the inner tube is burnished during drawing of the inner tube over the inner die tool or by rolling over the mandrel.
 11. The method according to claim 1, wherein a material of at least the inner tube or the outer tube is selected from a group consisting of a carbon steel, a low alloy steel and a high-alloy steel or a combination thereof.
 12. The method according to claim 1, wherein the first outer diameter of the inner tube prior to drawing of the outer tube and the inner tube extending therein together through the first drawing die is in a range from 6.25 mm to 6.45 mm, and after the common drawing is in a range from 6.08 mm to 6.28 mm, wherein the first outer diameter of the inner tube is reduced by the common drawing.
 13. The method according to claim 1, wherein the inner tube and the outer tube are made of the same material.
 14. The method according to claim 1, wherein the inner tube and the outer tube are made of different materials.
 15. The method according to claim 2, wherein the first outer diameter of the inner tube by drawing of the outer tube and the inner tube extending therein together through the first drawing die is reduced by at least 0.01 mm and by at most 0.3 mm.
 16. The method according to claim 3, wherein the inner die tool or the mandrel is made of steel with a polished surface, such that an inner shell surface of the inner tube is burnished during drawing of the inner tube over the inner die tool or by rolling over the mandrel. 